Well done World for getting through last night's talks.
I think you need to make a list of the pros and cons of having him live in the annex, and of starting the divorce sooner rather than later. It'll be very hard to have him there, and he'll probably want to wander into the house whenever he feels like it etc. But I can see that it may have certain advantages in terms of animals.
He's going to struggle to find a self contained flat to rent for less than six months-assured tenancies must legally be for at least six months. Short term will be very very expensive, That leaves a lodger type of scenario renting a room in someone's house, an airbnb type place.
I'd say definitely do your course as it'll be great for you in all sorts of social ways too. Play down the future career potential enormously!
Overall, it does concern me that divorce proceedings will get delayed. Partly because it leaves you treading water for so long which will be emotionally hard, and delays 'moving on' both practically and emotionally; and partly, because his circumstances may change, especially if OW gets pregnant, (or if he just gets more difficult and bolshy). Your settlement would then be much less I would imagine, but also, he'd want to immediately change all the plans and sell the house etc.regardless of what's best for you.
Also. he's likely to get tighter with money for you once this initial period passes- it usually happens this way... I'd be inclined, if I were in your shoes, to act decisively now, and keep control yourself of the timeframe in which everything happens. It could also be better financially to do it whilst dd is considered more dependent, not less.
Welfare wise for your dd, whenever you move house etc is going to be an upheaval for her. But it'll be the same, when she does A Levels. All you can do is keep the disruption to a minimum for her, when it does happen.
Only you can decide when divorce proceedings are best for you and DD. Just make sure it really is best for you two, and that you've not been manipulated into doing what's best for him.
Athens in August is going to be unbearable heat.
Don't cancel the cc. leave him in charge of everything, including dd for a week in September, and take yourself away for a relaxing time in the sun. (yes, he can organise his work hours so he has full charge of her)
Greek islands are lovely lovely in September! It may fuck up his holiday plans too, if you get your booking in first